arXiv subjects

L. N. Pfeiffer

Publications and source records attributed to L. N. Pfeiffer.

At least 109 records · Page 6Linked to original sources

arXiv2019

Effect of illumination on quantum lifetime in GaAs quantum wells

Low-temperature illumination of a two-dimensional electron gas in GaAs quantum wells is known to greatly improve the quality of high-field magnetotransport. The improvement is known to occur even when the carrier density and mobility remain unchanged, but what exactly causes it remains unclear. Here, we investigate the effect of illumination on microwave photoresistance in low magnetic fields. We find that the amplitude of microwave-induced resistance oscillations grows dramatically after illumination. Dingle analysis reveals that this growth reflects a substantial increase in the single-particle (quantum) lifetime, which likely originates from the light-induced redistribution of charge enhancing the screening capability of the doping layers.

cond-mat.mes-hall↗

arXiv2019

Quantum Hall stripes in high-density GaAs/AlGaAs quantum wells

We report on quantum Hall stripes (QHSs) formed in higher Landau levels of GaAs/AlGaAs quantum wells with high carrier density ($n_e > 4 \times 10^{11}$ cm$^{-2}$) which is expected to favor QHS orientation along unconventional $\left < 1\bar{1}0 \right >$ crystal axis and along the in-plane magnetic field $B_{||}$. Surprisingly, we find that at $B_{||} = 0$ QHSs in our samples are aligned along $\left < 110 \right >$ direction and can be reoriented only perpendicular to $B_{||}$. These findings suggest that high density alone is not a decisive factor for either abnormal native QHS orientation or alignment with respect to $B_{||}$, while quantum confinement of the 2DEG likely plays an important role.

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arXiv2019

Re-Symmetrizing the Broken Symmetry with Isotropic Hydrostatic Pressure

Recent progresses in condensed matter physics, such as graphene, topological insulator and Weyl semimetal, often origin from the specific topological symmetries of their lattice structures. Quantum states with different degrees of freedom, e.g. spin, valley, layer, etc., arise from these symmetries, and the coherent superpositions of these states form multiple energy subbands. The pseudospin, a concept analogy to the Dirac spinor matrices, is a successful description of such multi-subband systems. When the electron-electron interaction dominates, many-body quantum phases arise. They usually have digitized pseudospin polarizations and exhibit sharp phase transitions at certain universal critical pseudospin energy splittings. In this manuscript, we present our remarkable discovery of hydrostatic pressure induced degeneracy between the two lowest Landau levels. This degeneracy is evidenced by the pseudospin polarization transitions of the fragile correlated quantum liquid phases near Landau level filling factor $ν$ = 3/2. Benefitted from the constant hole concentration and the sensitive nature of these transitions, we can study the fine-tuning effect of the hydrostatic pressure of the order of 10 $μ$eV, well beyond the meV-level state-of-the-art resolution of other techniques.

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arXiv2019

Probing the Melting of a Two-dimensional Quantum Wigner Crystal via its Screening Efficiency

One of the most fundamental and yet elusive collective phases of an interacting electron system is the quantum Wigner crystal (WC), an ordered array of electrons expected to form when the electrons' Coulomb repulsion energy eclipses their kinetic (Fermi) energy. In low-disorder, two-dimensional (2D) electron systems, the quantum WC is known to be favored at very low temperatures ($T$) and small Landau level filling factors ($ν$), near the termination of the fractional quantum Hall states. This WC phase exhibits an insulating behavior, reflecting its pinning by the small but finite disorder potential. An experimental determination of a $T$ vs $ν$ phase diagram for the melting of the WC, however, has proved to be challenging. Here we use capacitance measurements to probe the 2D WC through its effective screening as a function of $T$ and $ν$. We find that, as expected, the screening efficiency of the pinned WC is very poor at very low $T$ and improves at higher $T$ once the WC melts. Surprisingly, however, rather than monotonically changing with increasing $T$, the screening efficiency shows a well-defined maximum at a $T$ which is close to the previously-reported melting temperature of the WC. Our experimental results suggest a new method to map out a $T$ vs $ν$ phase diagram of the magnetic-field-induced WC precisely.

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arXiv2019

Particle-Hole Symmetry and the Fractional Quantum Hall Effect in the Lowest Landau Level

We report on detailed experimental studies of a high-quality heterojunction insulated-gate field-effect transistor (HIGFET) to probe the particle-hole symmetry (PHS) of the FQHE states about half-filling in the lowest Landau level. The HIGFET was specially designed to vary the density of a two-dimensional electronic system under constant magnetic fields. We find in our constant magnetic field, variable density measurements that the sequence of FQHE states at filling factors nu = 1/3, 2/5, 3/7 ... and its particle-hole conjugate states at filling factors 1 - nu = 2/3, 3/5, 4/7 ... have a very similar energy gap. Moreover, a reflection symmetry can be established in the magnetoconductivities between the nu and 1 - nu states about half-filling. Our results demonstrate that the FQHE states in the lowest Landau level are manifestly particle-hole symmetric.

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arXiv2019

Effect of optically-induced potential on the energy of trapped exciton-polaritons below the condensation threshold

Exciton-polaritons (polaritons herein) offer a unique nonlinear platform for studies of collective macroscopic quantum phenomena in a solid state system. Shaping of polariton flow and polariton confinement via potential landscapes created by nonresonant optical pumping has gained considerable attention due to the degree of flexibility and control offered by optically-induced potentials. Recently, large density-dependent energy shifts (blueshifts) exhibited by optically trapped polaritons at low densities, below the bosonic condensation threshold, were interpreted as an evidence of strong polariton-polariton interactions [Nat. Phys. 13, 870 (2017)]. In this work, we further investigate the origins of these blueshifts in optically-induced circular traps and present evidence of significant blueshift of the polariton energy due to reshaping of the optically-induced potential with laser pump power. Our work demonstrates strong influence of the effective potential formed by an optically-injected excitonic reservoir on the energy blueshifts observed below and up to the polariton condensation threshold and suggests that the observed blueshifts arise due to interaction of polaritons with the excitonic reservoir, rather than due to polariton-polariton interaction.

cond-mat.quant-gas↗

arXiv2019

Direct measurement of the quantum geometric tensor in a two-dimensional continuous medium

Topological Physics relies on the specific structure of the eigenstates of Hamiltonians. Their geometry is encoded in the quantum geometric tensor containing both the celebrated Berry curvature, crucial for topological matter, and the quantum metric. The latter is at the heart of a growing number of physical phenomena such as superfluidity in flat bands, orbital magnetic susceptibility, exciton Lamb shift, and non-adiabatic corrections to the anomalous Hall effect. Here, we report the first direct measurement of both Berry curvature and quantum metric in a two-dimensional continuous medium. The studied platform is a planar microcavity of extremely high finesse, in the strong coupling regime. It hosts mixed exciton-photon modes (exciton-polaritons) subject to photonic spin-orbit-coupling which makes emerge Dirac cones and exciton Zeeman splitting breaking time-reversal symmetry. The monopolar and half-skyrmion pseudospin textures are measured by polarisation-resolved photoluminescence. The associated quantum geometry of the bands is straightforwardly extracted from these measurements. Our results unveil the intrinsic chirality of photonic modes which is at the basis of topological photonics. This technique can be extended to measure Bloch band geometries in artificial lattices. The use of exciton-polaritons (interacting photons) opens wide perspectives for future studies of quantum fluid physics in topological systems.

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arXiv2018

Interlayer Interactions and the Fermi Energy of Bilayer Composite Fermion Metals

When two 2D electron gas layers, each at Landau level filling factor $ν=1/2$, are close together a condensate of interlayer excitons emerges at low temperature. Although the excitonic phase is qualitatively well understood, the incoherent phase just above the critical layer separation is not. Using a combination of interlayer tunneling spectroscopy and conventional transport, we explore the incoherent phase in samples both near the phase boundary and further from it. In the more closely spaced bilayers we find the electronic spectral functions narrower and the Fermi energy of the $ν= 1/2$ composite fermion metal smaller than in the more widely separated bilayers. We attribute these effects to a softening of the intralayer Coulomb interaction due to interlayer screening.

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arXiv2018

Polariton-Enhanced Exciton Transport

The transport distance of excitons in exciton-polariton systems has previously been assumed to be very small ($\lesssim 1~μ$m). The sharp spatial profiles observed when generating polaritons by non-resonant optical excitation show that this assumption is generally true. In this paper, however, we show that the transport distances of excitons in two-dimensional planar cavity structures with even a slightly polaritonic character are much longer than expected ($\approx 20~μ$m). Although this population of slightly polaritonic excitons is normally small compared to the total population of excitons, they can substantially outnumber the population of the polaritons at lower energies, leading to important implications for the tailoring of potential landscapes and the measurement of interactions between polaritons.

cond-mat.quant-gas↗

arXiv2018

Unconventional anisotropic even-denominator fractional quantum Hall state in a system with mass anisotropy

The fractional quantum Hall state (FQHS) observed at a half-filled Landau level in an interacting two-dimensional electron system (2DES) is among the most exotic states of matter as its quasiparticles are expected to be Majoranas with non-Abelian statistics. We demonstrate here the unexpected presence of such a state in a novel 2DES with a strong band-mass anisotropy. The FQHS we observe has unusual characteristics. While its Hall resistance is well-quantized at low temperatures, it exhibits highly-anisotropic in-plane transport resembling compressible stripe/nematic charge-density-wave phases. More striking, the anisotropy sets in suddenly below a critical temperature, suggesting a finite-temperature phase transition. Our observations highlight how anisotropy modifies the many-body phases of a 2DES, and should further fuel the discussion surrounding the enigmatic even-denominator FQHS.

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arXiv2018

Wigner solids of wide quantum wells near Landau filling $ν=1$

Microwave spectroscopy within the Landau filling ($ν$) range of the integer quantum Hall effect (IQHE) has revealed pinning mode resonances signifying Wigner solids (WSs) composed of quasi-particles or -holes. We study pinning modes of WSs in wide quantum wells (WQWs) for $ 0.8\leν\le1.2$, varying the density, $n$, and tilting the sample by angle $θ$ in the magnetic field. Three distinct WS phases are accessed. One phase, S1, is phenomenologically the same as the WS observed in the IQHEs of narrow QWs. The second phase, S2, exists at $ν$ further from $ν=1$ than S1, and requires a sufficiently large $n$ or $θ$, implying S2 is stabilized by the Zeeman energy. The melting temperatures of S1 and S2, estimated from the disappearance of the pinning mode, show different behavior vs $ν$. At the largest $n$ or $θ$, S2 disappears and the third phase, S1A, replaces S1, also exhibiting a pinning mode. This occurs as the WQW $ν=1$ IQHE becomes a two-component, Halperin-Laughlin $\pone$ state. We interpret S1A as a WS of the excitations of $\pone$, which has not been previously observed.

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arXiv2018

Wigner solid pinning modes tuned by fractional quantum Hall states of a nearby layer

We study a bilayer system hosting exotic many-body states of two-dimensional electron systems (2DESs) in close proximity but isolated from one another by a thin barrier. One 2DES has low electron density and forms a Wigner solid (WS) at high magnetic fields. The other has much higher density and, in the same field exhibits fractional quantum Hall states (FQHSs). The WS manifests microwave resonances which are understood as pinning modes, collective oscillations of the WS within the small but finite ubiquitous disorder. Our measurements reveal a striking evolution of the pinning mode frequencies of the WS layer with the formation of the FQHSs in the nearby layer, evincing a strong coupling between the WS pinning modes and the state of the 2DES in the adjacent layer, mediated by screening.

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arXiv2018

High-mobility indirect excitons in wide single quantum well

Indirect excitons (IXs) are bound pairs of electrons and holes confined in spatially separated layers. We present wide single quantum well (WSQW) heterostructures with high IX mobility, spectrally narrow IX emission, voltage-controllable IX energy, and long and voltage-controllable IX lifetime. This set of properties shows that WSQW heterostructures provide an advanced platform both for studying basic properties of IXs in low-disorder environments and for the development of high mobility excitonic devices.

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arXiv2018

Critical Filling Factor for the Formation of a Quantum Wigner Crystal Screened by a Nearby Layer

One of the most fascinating ground states of an interacting electron system is the so-called Wigner crystal where the electrons, in order to minimize their repulsive Coulomb energy, form an ordered array. Here we report measurements of the critical filling factor ($ν_{C}$) below which a magnetic-field-induced, quantum Wigner crystal forms in a dilute, two-dimensional electron layer when a second, high-density electron layer is present in close proximity. The data reveal that the Wigner crystal forms at a significantly smaller $ν_{C}$ compared to the $ν_{C}$ ($\simeq 0.20$) in single-layer two-dimensional electron systems. The measured $ν_{C}$ exhibits a strong dependence on the interlayer distance, reflecting the interaction and screening from the adjacent, high-density layer.

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arXiv2018

Anomalous coupling between magnetic and nematic orders in quantum Hall systems

The interplay between different orders is of fundamental importance in physics. The spontaneous, symmetry-breaking charge order, responsible for the stripe or the nematic phase, has been of great interest in many contexts where strong correlations are present, such as high-temperature superconductivity and quantum Hall effect. In this article we show the unexpected result that in an interacting two-dimensional electron system, the robustness of the nematic phase, which represents an order in the charge degree of freedom, not only depends on the orbital index of the topmost, half-filled Landau level, but it is also strongly correlated with the magnetic order of the system. Intriguingly, when the system is fully magnetized, the nematic phase is particularly robust and persists to much higher temperatures compared to the nematic phases observed previously in quantum Hall systems. Our results give fundamental new insight into the role of magnetization in stabilizing the nematic phase, while also providing a new knob with which it can be effectively tuned.

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arXiv2018

Multivalley two-dimensional electron system in an AlAs quantum well with mobility exceeding $2\times10^6$ cm$^{2}$V$^{-1}$s$^{-1}$

Degenerate conduction-band minima, or `valleys', in materials such as Si, AlAs, graphene, and MoS$_2$ allow them to host two-dimensional electron systems (2DESs) that can access a valley degree of freedom. These multivalley 2DESs present exciting opportunities for both pragmatic and fundamental research alike because not only are they a platform for valleytronic devices, but they also provide a tool to tune and investigate the properties of complex many-body ground states. Here, we report ultra-high quality, modulation doped AlAs quantum wells containing 2DESs that occupy two anisotropic valleys and have electron mobilities peaking at $2.4\times10^6$ cm$^{2}$V$^{-1}$s$^{-1}$ at a density of $2.2\times10^{11}$ cm$^{-2}$. This is more than an order of magnitude improvement in mobility over previous results. The unprecedented quality of our samples is demonstrated by magneto-transport data that show high-order fractional quantum Hall minima up to the Landau level filling $ν=8/17$, and even the elusive $ν=1/5$ quantum Hall state.

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arXiv2018

Realization of a Valley Superlattice

In a number of widely-studied materials, such as Si, AlAs, Bi, graphene, MoS2, and many transition metal dichalcogenide monolayers, electrons acquire an additional, spin-like degree of freedom at the degenerate conduction band minima, also known as "valleys". External symmetry breaking fields such as mechanical strain, or electric or magnetic fields, can tune the valley-polarization of these materials making them suitable candidates for "valleytronics". Here we study a quantum well of AlAs, where the two-dimensional electrons reside in two energetically degenerate valleys. By fabricating a strain-inducing grating on the sample surface, we engineer a spatial modulation of the electron population in different valleys, i.e., a "valley superlattice" in the quantum well plane. Our results establish a novel manipulation technique of the valley degree of freedom, paving the way to realizing a valley-selective layered structure in multi-valley materials, with potential application in valleytronics.

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arXiv2018

Electron-Electron Interactions and the Paired-to-Nematic Quantum Phase Transition in the Second Landau Level

In spite of its ubiquity in strongly correlated systems, the competition of paired and nematic ground states remains poorly understood. Recently such a competition was reported in the two-dimensional electron gas at filling factor $ν=5/2$. At this filling factor a pressure-induced quantum phase transition was observed from the paired fractional quantum Hall state to the quantum Hall nematic. Here we show that the pressure induced paired-to-nematic transition also develops at $ν=7/2$, demonstrating therefore this transition in both spin branches of the second orbital Landau level. However, we find that pressure is not the only parameter controlling this transition. Indeed, ground states consistent with those observed under pressure also develop in a sample measured at ambient pressure, but in which the electron-electron interaction was tuned close to its value at the quantum critical point. Our experiments suggest that electron-electron interactions play a critical role in driving the paired-to-nematic transition.

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